Bimetallic Ions Codoped Nanocrystals: Doping Mechanism, Defect Formation, and Associated Structural Transition

Enhanced Energy Storage Performance by A-B Site Ambipolar Co-Doping in Antiferroelectrics

Antiferroelectric materials are promising to be used for power capacitive devices. To improve the energy storage performance, solid-solution and defect engineering are widely used to suppress the long-range order by introducing local heterogeneities. However, both methods generally deteriorate either the maximum polarization or breakdown electric field due to damaged intrinsic polarization or increased leakage. Here, we show that forming defect-dipole clusters by A-B site acceptor-donor co-doping in antiferroelectrics can comprehensively enhance the energy storage performance. We took the La-Mn co-doped (Pb_0.9Ba_0.04La_0.04)(Zr_0.65Sn_0.3Ti_0.05)O₃ (PBLZST) as an example. For co-doping with unequal amounts, high dielectric loss, impurity phase, and decreased polarization were observed. By contrast, La and Mn in an equal amount of co-doping can significantly improve the overall energy storage performance. An over 48% increasement in both the maximum polarization (62.7 μC/cm²) and breakdown electric field (242.6 kV/cm) was obtained in 1 mol % La and 1 mol % Mn equally co-doped PBLZST, followed by a nearly two-time enhancement in W_rec (6.52 J/cm³) compared with that of the pure matrix. Moreover, a high energy storage efficiency of 86.3% with an enhanced temperature stability over a wide temperature range can be achieved. The defect-dipole clusters associated with charge-compensated co-doping are suggested to contribute to an enhanced dielectric permittivity, linear polarization behavior, and maximum polarization strength compared with that of the unequal co-doping cases. The defect-dipole clusters are suggested to couple with the host, leading to a high energy storage performance. The proposed strategy is believed to be applicable to modify the energy storage behavior of antiferroelectrics.

Photocatalytic hydrogen generation using TiO2: a state-of-the-art review

This review is focusing on photocatalytic hydrogen (H2) production as a viable fuel. The limitations of different production methods for H2 generation and the importance of photocatalytic process are discussed, which renders this process as highly promising to meet the future energy crises. TiO2 is one of most effective material to generate the H2 via photocatalytic processes. Therefore, advantages of the catalyst over other semiconductors have been thoroughly analyzed. Starting from synthesis of TiO2 and factors affecting the whole process of photocatalytic H2 production have been discussed. Modifications for improvement in TiO2 and the photocatalytic reaction are critically reviewed as well as the mechanism of TiO2 modification has been described. Metal doping, non-metal doping, impurity addition and defect introduction processes have been analyzed and the comparison of experimental results is developed based on H2 production efficiency. A critical review of the literature from 2004 to 2021 concludes that H2 production as fuel using TiO2 photocatalytic method is efficient and environment friendly, which have potential for practical applications for H2 generation.

Engineering the Defects and Microstructures in Ferroelectrics for Enhanced/Novel Properties: An Emerging Way to Cope with Energy Crisis and Environmental Pollution

In the past century, ferroelectrics are well known in electroceramics and microelectronics for their unique ferroelectric, piezoelectric, pyroelectric, and photovoltaic effects. Nowadays, the advances in understanding and tuning of these properties have greatly promoted a broader application potential especially in energy and environmental fields, by harvesting solar, mechanical, and heat energies. For example, high piezoelectricity and high pyroelectricity can be designed by defect or microstructure engineering for piezo- and pyro-catalyst, respectively. Moreover, highly piezoelectric and broadband (UV-Vis-NIR) light-responsive ferroelectrics can be designed via defect engineering, giving rise to a new concept of photoferroelectrics for efficient photocatalysis, piezocatalysis, pyrocatalysis, and related cocatalysis. This article first summarizes the recent developments in ferroelectrics in terms of piezoelectricity, pyroelectricity, and photovoltaic effects based on defect and microstructure engineering. Then, the potential applications in energy generation (i.e., photovoltaic effect, H2 generation, and self-powered multisource energy harvesting and signal sensing) and environmental protection (i.e., photo-piezo-pyro- cocatalytic dye degradation and CO2 reduction) are reviewed. Finally, the outlook and challenges are discussed. This article not only covers an overview of the state-of-art advances of ferroelectrics, but also prospects their applications in coping with energy crisis and environmental pollution.

Hole-Pinned Defect Clusters for a Large Dielectric Constant up to GHz in Zinc and Niobium Codoped Rutile SnO2

High permittivity materials for a gigahertz (GHz) communication technology have been actively sought for some time. Unfortunately, in most materials, the dielectric constant starts to drop as frequencies increase through the megahertz (MHz) range. In this work, we report a large dielectric constant of ∼800 observed in defect-mediated rutile SnO₂ ceramics, which is nearly frequency and temperature independent over the frequency range of 1 mHz to 35 GHz and temperature range of 50-450 K. Experimental and theoretical investigations demonstrate that the origin of the high dielectric constant can be attributed to the formation of locally well-defined Zn²⁺-Nb⁴⁺ defect clusters, which create hole-pinned defect dipoles. We believe that this work provides a promising strategy to advance dipole polarization theory and opens up a direction for the design and development of high frequency, broadband dielectric materials for use in future communication technology.

Visible/near-infrared light absorbed nano-ferroelectric for efficient photo-piezocatalytic water splitting and pollutants degradation

The band structure of ferroelectrics can be modulated by mechanical stress induced piezoelectric polarization charges, and thus to promote the separation of photo-excited carriers, endowing photo-piezocatalysts with good performance in hydrogen production and pollutants degradation. However, the catalytic performance of these conventional photo-piezocatalysts is restricted since they mainly harvest UV light and generally have limited piezoelectricity. Here, in this study, by using self-propagation high-temperature synthesis process, highly piezoelectric gap-state-engineered nano relaxor ferroelectric at the morphotropic phase boundary, such as (Na_0.5Bi_0.5)TiO₃-Ba(Ti_0.5Ni_0.5)O₃ is synthesized for the first time and shows unprecedently light harvesting from UV to near-infrared (λ < 1300 nm). We demonstrate a significantly enhanced photo-piezocatalytic performance for this photo-piezocatalyst. A high hydrogen production rate of ~ 450 μmol g^-1 h^-1 is obtained and the decomposition of Rhodamine B dye is nearly completed after 20 min under irradiation and ultrasonic vibration. Moreover, an unprecedently efficient NIR-driven photocatalytic degradation of RhB is also demonstrated by using photo-piezocatalysts. This kind of novel multifunctional nano photo-piezocatalysts opens up new horizons to all-day available photo-piezocatalytic technology for a more efficient use of multisource energies from environment.

Structural Phase Transition and Metallization of Nanocrystalline Rutile Investigated by High-Pressure Raman Spectroscopy and Electrical Conductivity

We investigate the structural, vibrational, and electrical transport properties of nanocrystalline rutile and its high-pressure polymorphs by Raman spectroscopy, and AC complex impedance spectroscopy in conjunction with the high-resolution transmission electron microscopy (HRTEM) up to ~25.0 GPa using the diamond anvil cell (DAC). Experimental results indicate that the structural phase transition and metallization for nanocrystalline rutile occurred with increasing pressure up to ~12.3 and ~14.5 GPa, respectively. The structural phase transition of sample at ~12.3 GPa is confirmed as a baddeleyite phase, which is verified by six new Raman characteristic peaks. The metallization of the baddeleyite phase is manifested by the temperature-dependent electrical conductivity measurements at ~14.5 GPa. However, upon decompression, the structural phase transition from the metallic baddeleyite to columbite phases at ~7.2 GPa is characterized by the inflexion point of the pressure coefficient and the pressure-dependent electrical conductivity. The recovered columbite phase is always retained to the atmospheric condition, which belongs to an irreversible phase transformation.